This disclosure pertains to optical imagers and in particular to an anamorphic relayed imager having multiple rotationally symmetric powered mirrors.
Demand for imaging systems that provide wide area surveillance is increasing. Wide area surveillance can be performed at an elevated position relative to ground surface. Wide area surveillance can be used in various applications such as, for example, on an unmanned aerial vehicle (UAV) platform, an aircraft, or a satellite for target recognition. Wide area surveillance can be performed at various wavelength ranges depending on the desired application. The wavelength ranges of interest include short wavelength infrared radiation (SWIR) in the wavelength range between approximately 1 μm and 3 μm, mid wavelength infrared radiation (MWIR) in the wavelength range between approximately 3 μm and 5 μm, and long wavelength infrared radiation (LWIR) in the wavelength range between approximately 8 μm and 12 μm.
Imaging systems use an image sensing device for sensing radiation collected by a series of imaging optics including one or more lenses, one or more mirrors, or both. Typically, the image sensing device is a focal plane array (FPA), which includes an array or matrix of radiation sensing pixels, positioned at a focal plane of the optics.
Conventional FPAs generally include square pixels such that when used with conventional optical systems, a square ground sample distance (GSD) or footprint is projected when viewing an object at or near nadir from an elevated position, i.e., when viewing nearly straight down at an angle of approximately 0 deg. relative to nadir direction from a viewing platform (e.g., a UAV, a satellite, etc.). However, when viewing at an angle that is extremely off-nadir (e.g., approaching the local horizon), the square angular subtense of the square FPA pixel becomes elongated in the elevation direction to an extreme rectangular GSD or footprint. This effectively reduces the resolution of the EPA sensor in that direction. For example, when viewing at an elevation angle of approximately 45 deg. from nadir direction, the elevation GSD is double the elevation GSD when viewing at nadir direction (assuming a flat earth surface). When viewing at an elevation angle of approximately 60 deg. from nadir direction, the elevation GSD is quadruple the elevation GSD when viewing at nadir direction (assuming a flat earth).
In order to limit the impact on resolution for viewing at greater angles from the nadir direction, a focal length of the imaging optics is approximately doubled in the along-scan (A-S) or elevation direction relative to a focal length in the cross-scan (C-S) direction perpendicular to the A-S direction. In this way, the angular subtense of the FPA square pixel is halved (divided by two) in the A-S direction. As a result, a smaller GSD can be maintained at large viewing angles from nadir direction. The difference in focal length between a plane in the imaging optics in the A-S direction and a plane in the imaging optics in the C-S direction is termed anamorphism. Most conventional optical systems have little or no anamorphism.
Conventional optical systems that exhibit any useful degree of anamorphism do so by using cylindrical surfaces on mirrors, lenses, such as in cinematographic recording systems (e.g., CINEMASCOPE, PANAVISION, etc.). However, such cylindrical surfaces are difficult to make and test.
Currently, there are no practical optical systems useful in an aerospace environment which achieve (and thus employ) such anamorphism to maintain good ground resolution. By practical optical system, it is meant an optical system with a relatively large FOV (FOV greater than about 3 deg.) to be useful, an optical system that maintains good image quality over the entire FOV, an optical system that is all-reflective and present essentially no-sensing wavelength restrictions, or an optical system that provides 100% cold-shielding and thus allow the use of cold IR FPAs, or any combination of two or more thereof.
Therefore there is a need for anamorphic imaging systems that provide one or more of the above features.